The present invention relates to an inductive ignition apparatus for a combustion engine, having a measurement device for ascertaining the ionization current at the spark plug of each cylinder, and having for each spark plug an ignition coil device which forms the ignition high voltage, operates on the transformer principle, and has a primary and a secondary winding, and through whose secondary winding the ionization current flows.
Accurate diagnostic systems which allow conclusions to be drawn as to the combustion process are necessary in order to operate combustion engines at high efficiency and to meet stringent requirements in the area of on-board diagnosis. These diagnostic systems should moreover preferably be economical. Conventional diagnostic systems take important information about the progress of combustion directly from the combustion chamber of a combustion engine (internal combustion engine). This is done by so-called ionization current measurement, in which during a combustion cycle, the spark plug first performs its actual task, i.e., igniting the combustion mixture, and is then used for a further function by being utilized as a sensor with which the ionization current is measured. This is advantageous because no space is needed in the combustion chamber for additional sensors. Ionization current measurement is based on the principle that ions are produced during the combustion of the fuel-air mixture. This ionization is the result of a variety of mechanisms which influence the typical profile of the ionization current and therefore provide information regarding specific parameters of the combustion process, etc. When a voltage is applied to the electrodes of the spark plug for ionization current measurement, electrons and ions present in the combustion chamber are moved in the corresponding direction of the electric field, thus creating a current which is carried by these charge carriers. This current represents the aforementioned ionization current. When the ionization current measurement method, method is used with an inductive ignition apparatus which has an ignition coil device that operates according to the transformer principle and has a primary winding and a secondary winding, there exists, because of the relatively high secondary inductivity, the disadvantage that the spark duration of the spark plug is difficult to control; this can impede the measurement. In addition, because of the relatively high secondary inductivity in the ionization current path, it is possible to transfer only relatively low frequencies which, for example, are not sufficient for reliable knock detection.
The ignition apparatus according to the present invention has an advantage that as a result of a switch which short-circuits the primary winding of the ignition coil device for the duration of the ionization current measurement, the residual energy in the magnetic circuit of the ignition coil is dissipated on the primary side, i.e., is converted into heat energy and in that respect no longer drives the ignition spark, so that the latter is extinguished very quickly and reproducibly at the desired time. Short-circuiting of the primary side of the ignition coil device moreover shifts the limit frequency of the secondary side of the ignition coil considerably upward, so that any knock vibrations of the combustion engine that might occur can be observed in undamped fashion as an undesirable operating state, since the knock vibrations do not entail significant ionization current profiles.
It is particularly advantageous if the switch path of the switch has very low resistance in the closed state. The primary circuit of the ignition coil device thus has a much lower resistance than the secondary circuit, so that the ignition spark is quickly extinguished.
In particular, the switch can be configured as a field-effect transistor (FET) which possesses a low-resistance switch path at low flux voltages.
Lastly, it is advantageous if the measurement device has a control device which closes the switch preferably periodically at the desired spark termination, at least for the duration of the entire ionization current measurement. In the case of a field-effect transistor, this is accomplished by activating it accordingly.